Upper Interface Engineering Between Perovskite and Electron Transport Layer Toward Efficient and Stable Inverted Perovskite Solar Cells

Inverted (p-i-n) perovskite solar cells (PSCs) have emerged as a promising photovoltaic technology due to their excellent stability and compatibility with tandems. However, their performance is critically limited by the perovskite/electron transport layer (ETL) upper interface. This interface governs electron extraction efficiency, ion migration, and non-radiative recombination, directly affecting open-circuit voltage (Voc) and fill factor (FF). Moreover, interfacial defects, energy level misalignment, and chemical instability at this junction often lead to hysteresis and degradation, hindering commercialization efforts. This review systematically examines interfacial modification strategy through a materials classification approach by four categories: ammonium salts, organic molecules, polymers, and inorganic materials. How these materials optimize charge transport, suppress ion migration, and enhance interfacial adhesion is thoroughly discussed. A systematic understanding of structure-property relationships at this critical interface is elucidated. Besides, characterization techniques for probing interfacial phenomena is summarized. Finally, the challenges and opportunities are elucidated at this underexplored interface. These insights position upper interface engineering as the next frontier in realizing commercially viable, high-performance inverted PSCs.